1. Field of the Invention
The present invention relates to mechanism and a method for detecting the rhythmic voltage fluctuations or electrical signals generated in the brain and displaying such signals in viewable form, and also for storing such signals electronically so that they can be reproduced and displayed in viewable form at a desired time.
2. Prior Art
It has been proposed to monitor the brain waves or electrical brain activity of an anesthetized patient to indicate the depth or level of anesthesia, the object being to prevent too little or too much of the anesthetic agent from being administered to the patient. In addition, it is known that the level of electrical bran activity is affected by a substantial, sudden decrease in blood flow through the brain, and by a substantial decrease in oxygen supplied to the brain regardless of cerebral blood flow, which always are of concern while the patient is under the effect of an anesthetic agent. Accordingly, it has been proposed to continuously monitor the patient's electrical brain activity during a surgical operation and recovery.
The primary mechanism used to monitor electrical brain activity is the conventional electroencephalograph (EEG) which, unfortunately, has not proven to be practical for routine use in the operating or recovery room. In a conventional EEG, several, such as 16, electrodes attached to a patient's scalp detect the very small voltage fluctuations generated in the brain. The small voltage fluctuations are amplified and are separately displayed. Typically, 16 different EEG outputs are provided, each indicating the detected voltage difference between a selected pair of electrodes.
Interpretation of the many EEG outputs can require substanial study even by experienced EEG specialists which are expensive and in short supply. To allow time for this study, a permanent record of the EEG outputs is essential, which has been accomplished by use of a chart recorder having several pens deflected in response to the EEG ouputs and making tracings on a chart moved past the pens. So that the fluctuations are prominantly displayed on the chart, the chart is moved past the pens at fairly high speed, such as in the range of 15 mm to 60 mm per second. If the electrical brain activity is monitored over any substantial period, a voluminous and complicated paper record is generated.
Modified EEG-type devices have been proposed to overcome the expense and inconvenience of long-term brain wave monitoring by a conventional EEG. One such device is the "cerebral function monitor" described generally in an article titled "EEG Processing By The Cerebral Function Monitor (CFM)" (Ann. Anesth. Franc., 3:170-174, 1979) by the inventor of that device D. E. Maynard. The Maynard cerebral function monitor also is the subject of U.S. Pat. No. Re. 28,214 and British Pat. Nos. 1,247,491 and 1,247,492.
In general, the Maynard cerebral function monitor uses only two electrodes attached to the patient's scalp to detect the small voltage fluctuations generated in the brain. Only one electrical brain activity output signal is provided, with frequency components below about 2 hertz and above about 14 hertz filtered out. The remaining 2 hertz to 15 hertz signal is amplified, with the higher frequency components being amplified to a greater degree than the lower frequency components. The single filtered and amplified signal is displayed by use of a chart recorder, but the chart is moved at slow speed, such as several millimeters per minute, so that the pen oscillations are closely packed in a broad band. The top of the band indicates the maximum amplitude of electrical brain activity at a specific point in time and the bottom of the band indicates the minimum amplitude of the electrical brain activity at about the same time.
Initially Maynard proposed that his cerebral function monitor be used as a check on the validity of conventional EEG tracings taken intermittently over a long period. Various other uses for the cerebral function monitor have been suggested including its use as an indicator of the depth or level of anesthesia and to give a warning of a substantial, sudden decrease in cerebral blood flow which should be indicated by a corresponding decease in the amplitude of the cerebral function monitor output. See, for example, the article titled "General anaesthesia and changes on the cerebral function monitor" (Anaesthesia 33:157-164, 1978) by Michel Dubois, T. M. Savege, T. M. O'Carroll and M. Frank.
Other modified EEG-type devices are described in an article titled "An Inexpensive Device for Analyzing and Monitoring the Electroencephalogram" (Anesthesiology 50:456-460, 1979) by Robert A. Fleming and N. Ty Smith.